Twilight found a spell whose exact effects are unknown. All she knows about it is that you need three alicorns to cast it. Celestia and Luna agree to help her with the spell, curiosity clearly overtaking the princesses. The result... a small portion of each of their magical energies are transferred into a poor, random, passerby who just happened to be in the area, turning them into an alicorn.

Normally one has to prove themselves worthy of being an alicorn, but the overworked princesses see this as an opportunity. In order for Stardust to prove herself a worthy princess, even if she unwilling was dubbed one, she must complete various royal duties for the previous princesses to prove she has what it takes.

If all goes well Celestia might get the vacation she had wanted for the last thousand years...

(Note: My alicorn character is pretty much useless in terms of abilities, almost no actual magic, terrible at flight, and not very strong... this abomination of a story is meant as a comedy after all!)